Thermoplastic vulcanizates (“TPVs”) are a fine dispersion of highly vulcanized rubber in a continuous phase of a polyolefin. TPVs are traditionally made by blending a rubber with a semi-crystalline polyolefin under conditions that allow for the dynamic vulcanization of the rubber. The result is a material comprised of a continuous plastic phase formed by the polyolefin and interspersed with discrete, crosslinked rubber particles, which form a rubber phase. TPVs have the benefit of the elastomeric properties provided by the rubber phase, with the processability of thermoplastics.
TPVs have been widely used in applications that are subject to compression forces, including, for example, gaskets, grips, seals, stoppers, and damping pads. In such applications, it is generally preferable to lower the compression set of the TPV in order to improve the article's usefulness without adversely affecting other material characteristics, including processability. Traditionally, efforts to improve compression set in TPVs have focused on achieving a higher rubber cure state in the rubber phase. See, C. P. Rader, “Elastomeric Alloy Thermoplastic Vulcanizates”, in HANDBOOK OF THERMOPLASTIC ELASTOMERS 86, 96-99 (B. M. Walker and C. P. Rader eds. Van Nostrand Reinhold, NY, 1988). The plastic phase, which has essentially no inherent elastic recovery, has not been considered as a suitable variable in affecting compression set of TPVs. Rather, the thermoplastic characteristic of the plastic phase is generally viewed as a limiting factor in relation to the compression set of TPVs.
It is generally accepted that, while maintaining a suitable compression set, the upper service temperature of a TPV will directionally relate to the melting point of the plastic phase; namely that using polyolefins having higher melting points in the plastic phase will afford improved compression set in TPVs at higher temperatures. This result would be expected insofar as it would be expected that a plastic phase having a higher melting point would afford improved high temperature (70° C. and 100° C.) elastic recovery of associated TPVs, by virtue of the plastic phase's increased resistance to thermal deformation at higher temperatures. Isotactic homopolypropylene (Tm˜165° C.) has been widely adopted as the polyolefin of choice in high temperature TPV applications, for, among other reasons, its high melting point. See, T. Abraham and C. McMaham, “Thermoplastic Elastomers; Fundamentals and Applications”, in RUBBER COMPOUNDING ; CHEMISTRY AND APPLICATIONS, 212 (B. Rodgers ed., Marcel Dekker, Inc., New York, N.Y. 2004).
While it is known to use a variety of different polyolefins, including those having high and low melting points, in TPVs, it is generally taught that TPV compositions may include any of the variety of suitable polyolefins, polypropylene and polyethylene being exemplary, in combination with a variety of different elastomers, EPDM rubber and SB rubber being exemplary, without regard to the melting point characteristic of the polyolefin and the effect of polyolefin melting point on the compression set of the resulting TPV.
For example, U.S. Pat. No. 4,250,273 describes a blend of uncured or partially cured mixtures of SB rubber, 1-olefin polymers or copolymers and highly saturated elastomers. However, there is no discussion on selecting between 1-olefin polymers to improve high temperature compression set of the resulting TPV. Further, for processability reasons, this patent discloses uncured or partially cured mixtures rather than fully cured TPVs.
U.S. Pat. No. 4,340,684 describes thermoplastic elastomeric blends of 1-olefin polymers, SB rubbers and highly saturated elastomers and is a divisional patent of the previously mentioned U.S. Pat. No. 4,250,273. Similarly, U.S. Pat. No. 4,350,795 is also a divisional of that same patent. All three of these patents relate to tri-blends including 1-olefin polymer or copolymer and SB rubber. U.S. Pat. No. 4,385,142 is related to these previously described patents, but further include from 5 to 50 parts by weight of bitumen. None of these references disclose compositions having plastic phases selected on the basis of melting point for the purpose of improving TPV compression set.
U.S. Pat. No. 4,927,882 describes thermoplastic elastomer compositions comprising SB rubbers in a co-continuous matrix of SEBS and polypropylene. No reference is made to the substitution of polypropylene with polyolefins having a lower melting temperature for purposes of improving compression set.
U.S. Pat. No. 4,202,801 describes dynamically partially cured blends of monoolefin copolymer rubbers and polyolefin resins. This reference discloses the use of both high and low melting point temperature polyolefins (polypropylene and polyethylene being exemplary) in the partially cured blends, but does not teach distinguishing between the polyolefins to improve compression set of the resulting TPV.
U.S. Pat. No. 4,104,210 describes thermoplastic elastomeric compositions comprising blends of highly unsaturated diene rubber and thermoplastic olefin resins. While a variety of olefin resins are described, including those having high and low melting point temperatures as defined herein, there is no teaching to select between the olefin resins to improve compression set in the composition.
It would be preferable in many instances to improve the compression set of TPVs by taking selective advantage of the distinctive characteristics between polyolefins in the plastic phase, as reflected in their respective melting points.
Finally, the use of processing agents, most notably paraffinic oil, naphthenic oil, and aromatic process oils, to aid in the processability of TPV compositions is well documented (for example in U.S. Pat. No. 6,667,364). Such processing oils reduce viscosity during blending of the plastic and rubber TPV constituents, thus aiding the dispersion of the rubber phase in the continuous plastic phase. Further, the processing oils may be absorbed in the rubber phase of the TPV, thereby increasing the volume of material. By increasing the volume of material using relatively low cost processing oils, overall cost can be reduced. In many instances, it may be preferable to substantially saturate the TPV with processing oil in order to maximize volume and processability. However, over-saturation of the TPV with processing oil can result in oil bleed. It would be advantageous, therefore, to be able to determine the amount of processing oil to add in order to achieve substantial saturation of the TPV without oil bleed.